Influence of Defects on the Electrical Characteristics of Mercury-Drop Junctions:  Self-Assembled Monolayers of n-Alkanethiolates on Rough and Smooth Silver

Abstract

This paper compares the structural and electrical characteristics of self-assembled monolayers (SAMs) of n-alkanethiolates, SCn (n = 10, 12, 14), on two types of silver substrates: one used as-deposited (AS-DEP) by an electron-beam evaporator, and one prepared using the method of template-stripping. Atomic force microscopy showed that the template-stripped (TS) silver surfaces were smoother and had larger grains than the AS-DEP surfaces, and reflectance-absorbance infrared spectroscopy showed that SAMs formed on TS substrates were more crystalline than SAMs formed on AS-DEP substrates. The range of current densities, J (A/cm2), measured through mercury-drop junctions incorporating a given SAM on AS-DEP silver was, on average, several orders of magnitude larger than the range of J measured through the same SAM on TS silver, and the AS-DEP junctions failed, on average, 3.5 times more often within five current density-voltage (J-V) scans than did TS junctions (depending on the length of the alkyl chains of the molecules in the SAM). The apparent log-normal distribution of J through the TS junctions suggests that, in these cases, it is the variability in the effective thickness of the insulating layer (the distance the electron travels between electrodes) that results in the uncertainty in J. The parameter describing the decay of current density with the thickness of the insulating layer, beta, was either 0.57 A-1 at V = +0.5 V (calculated using the log-mean of the distribution of values of J) or 0.64 A-1 (calculated using the peak of the distribution of values of J) for the TS junctions; the latter is probably the more accurate. The mechanisms of failure of the junctions, and the degree and sources of uncertainty in current density, are discussed with respect to a variety of defects that occur within Hg-drop junctions incorporating SAMs on silver.

Mechanism of Rectification in Tunneling Junctions Based on Molecules with Asymmetric Potential Drops

“…25 The Ag TS electrodes have a lower surface roughness (root-meansquare roughness = 1.2 ± 0.1 nm measured over an area of 25 µm 2 ) than substrates used as-deposited by e-beam evaporation (AS-DEP, root-mean-square roughness = 5.2 ± 0.4 nm measured over an area of 25 µm 2 ). 25 The maximum grain size was ~1 µm 2 on the Hg top-electrode are required to obtain stable, non-shorting junctions). 25,53,54 We found that the shape and surface roughness of the cone-shaped tips depend on the operator.…”

“…2), thin-area defects cause a much greater deviation between the predicted and measured values of J than thick-area defects. 25 Thin-area defects lead to high observed values of J, and these anomalously high values of J can dominate the observed transport of charge through a junction to a disproportionate extent, relative to their area. By contrast, thick-area defects decrease the observed value of J, but only in (approximately) direct proportion to their area.…”

“…24 The measured tunneling current is sensitive to, or may even be dominated by, defects in the junctions that cause variations of the distance between the top-and bottom-electrodes. 25 (2) Figure 2 shows several types of defects that may be present in the tunneling junctions.…”

“…25 The maximum grain size was ~1 µm 2 on the Hg top-electrode are required to obtain stable, non-shorting junctions). 25,53,54 We found that the shape and surface roughness of the cone-shaped tips depend on the operator. 55 These variations of the cone-shape tips introduce ambiguities in the measurement of current density.…”

Molecules on Si: Electronics with Chemistry

“…[96,132,133] To control both transport and dipole effects, it seems therefore best to start from a transport quality monolayer that passivates the surface chemically and electronically, and then add polar (or other functional) moieties that do not affect the integrity of the interface. [49,[134][135][136] …”